Telecommunications systems transmit signals between user equipment, e.g., telephones, radios, and computers, over a network. Conventional telecommunications systems include, but are not limited to, the public switched telephone network (PSTN), the Internet, wireless networks, and cable television networks. These networks typically include transmission media such as coaxial cable, copper wires, optical fibers, and wireless links, e.g., radio and satellite communications.
Conventionally, transmission media carry the signals over the network in channels. In carrying signals, the channels may degrade or otherwise negatively impact the quality of the signals generated by one user for transmission to another user due to characteristics of the channel. For example, “multipath fading” is one source of degradation in signals of the channel in a telecommunications network. In wireless networks, multipath fading is often experienced due to the reception of signals that traverse different propagation paths. The multiple propagation paths are typically produced by atmospheric refraction and layering. Alternatively, the multipath signals may be produced by reflections from ground clutter near a radio receiver. Other systems also may suffer from multipath fading. For example, hybrid fiber-coax systems typically experience multipath fading due to impedance mismatches between various network components.
Time dispersion of a multipath channel is often characterized by the root mean squared (rms) time delay spread (στ). This quantity is defined as the square root of the second central moment of the power delay profile. In the frequency domain, fading is often characterized as “flat” or “frequency selective” based on the relationship between rms delay spread and the duration of the modulated digital signal (TSYM). Flat fading occurs when the rms time delay spread of the received signal is small enough to not cause significant intersymbol interference (ISI). A common relationship used in industry is that στ<0.1 TSYM to characterize a channel as flat fading. Conversely, a channel may be considered frequency selective if the στ>0.1 TSYM. There is not a sharp distinction between the two types of channels as these mathematical relationships imply, but they serve as a starting point for further analysis. If a communications channel is frequency-selective, the resultant intersymbol interference produces a system bit error rate floor.
During the design of a particular network, circuits are often included in an attempt to overcome these problems to produce a signal at a receiver that fairly represents the signal generated at a transmitter. For example, an equalizer is often used to compensate for a frequency-selective channel in a digital communications system. The equalizer reduces the intersymbol interference present at its output port, thereby lowering the system bit error rate produced by such interference. In effect, the equalizer acts as an inverse filter of the communications channel.
For an equalizer to be effective, the equalizer settings, e.g., the equalizer coefficients, are selected so as to compensate for the degrading effects of the channel at the time of transmission. In many systems, the channel characteristics are generally unknown and may change over time. Adaptive equalizers are commonly used to compensate for a time varying channel transfer function. In systems that process a continuous signal, the adaptation of equalizer coefficients is well understood by those skilled in the art. However, single adaptive equalizers, when employed in Time Division Multiple Access systems, may not be effective in reducing intersymbol interference since the channel may have significantly changed since the last transmission from a particular user. In fact, if a transmission over the channel is equalized with the equalizer settings used for a prior transmission, the performance of the system may be degraded more than if no equalization were performed at all.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improvements in compensating for multipath fading in telecommunications systems.